Graft copolymers have been formed by techniques in which a polymeric substrate is wet out with a solution of a monomer and subjected to ionizing radiation to cause the monomer to polymerize in grafted form on the substrate. One example of this technique is illustrated in Cline et al, U.S. Pat. No. 3,090,664 in which an unsaturated organic acid or salt is grafted onto a nitrogen containing polymer substrate. According to this patent, the bulk of the irradiation received by the wet fabric comprises photons at wave lengths less than 3000 A. Optional photoinitiators activated by ultraviolet light are indicated by the patent as a means for reducing the ultraviolet light exposure required for grafting. Such photoinitiators are disclosed as being added in the liquid form together with the monomer.
The process of the Cline et al patent has a number of disadvantages. Because the monomer is added in a liquid form and penetrates deeply into the polymeric substrate, it tends to homopolymerize within the substrate as well as on the surface. Thus, a relatively thick uneven layer of homopolymer is deposited. In certain instances, such as grafting of monomers for permanent press characteristics, such a thick coating results in undesirable tactile properties such as roughness, stiffness and poor draping. In addition, where the polymeric substrate is a fiber, the excess liquid monomer tends to form bridges across fibers within the fabric during polymerization leading to unsightly spot welding which also contributes to fabric stiffness.
Another disadvantage to graft polymerization using a liquid monomer is that there is a substantial waste of excess monomer which is homopolymerized on the substrate surface. Also, addition of the photoinitiator in liquid form also is wasted.
Irradiation at the relatively high energy disclosed in the Cline et al patent (less than 3000 A.) produces detrimental effects upon the polymeric substrate. Noticeable effects are discoloration of the substrate and reduction in tensile strength.
Another disadvantage of the above technique is that when the optional photoinitiator is employed, it is added as a liquid. Such photoinitiators, which are normally colored, tend to stain or discolor the fabric when added in this form.